In structures where the integrity of some parts must be guaranteed in service, for example in aircraft structural parts, the sensitive parts, at least for their critical and vulnerable portions, should not be subjected to shocks that may damage them and which would have the effect of decreasing their mechanical characteristics in the more or less long term.
In particular, certain portions of parts, which will be not visible and protected when the structure is in service, appear vulnerable during manufacture, for example due to a possibility of a dropped tool or a person accidentally stepping on a vulnerable area.
This problem must be taken into account in the case of composite materials comprising a stacking of plies adhering to each other via a resin, where a shock can lead to plies becoming separated inside the material, a local delamination, which is not visible from the outside and requires the implementation of means to verify the internal structure of the material in order to be detected and characterized.
A first known method for guaranteeing the integrity of the parts consists of protecting the parts, temporarily or permanently, by devices able to absorb the energy, within certain limits, during an impact. Many devices, most often adapted to specific situations, therefore use materials made from elastomer forming mattresses or protective panels made from wood or metal, which temporarily or permanently cover the area and the elements to be protected.
In certain situations the mass or dimensions of such protective devices make them difficult to use.
Another known method consists of preparing the parts so as to ensure that when a shock occurs the event will be detected subsequently during an inspection.
Thus certain methods utilize parts mounted in the areas at risk, designed to be damaged by absorbing the energy of a shock in order to protect the structure and fixed on the structure so that they can be replaced if necessary. Such parts, known as sacrificial because they are designed to be sacrificed so as to protect another part, present the defect that they must be adapted to each model of the part to be protected, which in practice limits use to cases in which a specific risk has been identified.
It is known, for example, to protect the tops of stiffeners of structural panels by placing elements enveloping the stiffener at their summits, as described in patent application FR2932707. In the event of a shock the enveloping element is damaged protecting the stiffener and can be replaced. In this case, when it is necessary to remove the damaged element precautions must be taken so that the protected structure is not then damaged.
Other methods have the sole purpose of detecting when a shock has actually occurred on a part, showing the location of the impact. For this purpose it is proposed, for example in patent GB 2 194 062, to use paints incorporating microcapsules of a coloring product, not visible in the encapsulated state, but which becomes observable in the visible range when the dye is released as a result of a shock having locally broken the microcapsules, giving a good indication of the extent of the impacted area.
An inconvenience of this method is the fact that it is difficult in this case to assess the energy of the impact, and therefore how great the risk of damage is; for that it is, in practice, necessary to carry out a detailed inspection of the impacted area, an area for which, in addition, the paint does not provide any mechanical protection against shocks.
Moreover, such paints are costly, more sensitive to abrasion that standard paints and, if the precaution of repainting an impacted area with a paint of this type is not taken, shocks will no longer be detected.